/Engine/Source/Editor/UnrealEd/Public/SnappingUtils.h
Snaps a delta drag movement to the nearest vertex
true if anything was snapped
static bool SnapDragLocationToNearestVertex ( const FVector & BaseLocation, FVector & DragDelta, [FLevelEditorViewportClient](API\Editor\UnrealEd\FLevelEditorViewportClient) * ViewportClient, bool bIsPivot )